Wednesday, June 5, 2013

Uncertainty is the Bane of the Risk Assessment Process

Almost 20 years ago I had the privilege of presenting testimony before the President’s Commission on Risk Assessment.  I used that opportunity to describe and bemoan the relative nascent state-of-the-science and how uncertainties in both the toxicology and exposure assessment of risk assessment were really limiting the utility and ultimately the credibility of the process.   A measure of our progress might be seen in fact that the title of that talk and the title of this week’s blog are identical.   That talk from almost 20 years ago goes into the subject in a lot more detail.   If any of you reading this ask me for it I will send you the PowerPoint slides from that presentation (

This is not to say that no progress have been made; indeed, a lot has happened in twenty years on the exposure side of things.   Some really great tools have been developed by the volunteers within the American Industrial Hygiene Association which have been made generally available as books and freeware.    Go to:
to get some of these models and go to the AIHA Press:
to see the books on Exposure Assessment Strategies, Risk Assessment Principles and Mathematical Modeling.   Even with these tools a ton of work still needs to happen on the exposure side of risk to reduce the uncertainty; however, I must say that we have come a long way.

From my perspective, even more work needs to happen in toxicology.   Clearly, advanced techniques have been developed but from my perspective, available data that provides confident information on what is happening in human tissue at environmentally relevant concentrations of chemical exposure has not happened.   Most of the current occupational exposure limits (OELs) are based on the current toxicity base which uses the responses of animals at high doses in order to estimate relatively “safe” or allowable levels of exposure to humans at relatively low exposure.     See previous blogs for a more detailed discussion of this subject.

So what might be done with all this uncertainty?   Is the risk assessment process so laden with uncertainty as to be worthless?  I think not.  Indeed, at almost any level, a rational assessment of the health risk posed by chemical exposure is better than not doing it.   For me, the challenge is appropriately describing that uncertainty for the various stakeholders in the process.    Indeed, the suitable description of uncertainty would hopefully point the way to its reduction.   

Properly handed using the upper end of the uncertainty – for example, the 95th upper bound percentile -as the measure of risk will reasonably bias the analysis toward overestimating and not underestimating the risk.   Thus, if one is able to shrink the range of uncertainty (with more data) the 95th percentile upper bound will come down as well which means we are properly trading data for conservatism in the spirit of a precautionary approach.

For many years I used Monte Carlo uncertainty analysis for the assessment of risk which allows for everyone to see the specific uncertainty that exists and feeds the various parts of the assessment.   It gives the stakeholders a good view of where the uncertainties exist and how additional information might truly help the evaluation and the subsequent decisions.   This and a simpler form of uncertainty analysis will the subject of a future blog.

Given the level of uncertainty in our current set of OELs, my suggestion is that we quantitative describe the uncertainty within these values as part of the documentation of any OEL.    Doing so will typically reveal a relatively large range of uncertainty which some may find disconcerting; however, I believe that disclosing it is a critical part of the integrity of the process.   More about this critical issue of documenting OELs will happen in a future blog as well.   

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